The long-term goal of this research is to understand how humans continuously stabilize their bodies while walking. Balance is necessary to prevent falling, an issue relevant to the rehabilitation of individuals with vestibular and other sensory impairments, as well as elderly adults who suffer from decreased sensory function. Although a great deal is known about postural balance while standing, less is known about balance while walking, because translation of the body makes it more difficult to make measurements and apply perturbations. We will develop a new experimental device that will make it possible to manipulate balance during walking, so that the resulting human control responses can be measured. We will then use this device to study common gait adaptations such as to step length and step width, and their impact on stability. These adaptations will be studied in both young and elderly subjects. The Specific Aims of this project are: 1. To design and fabricate an active device that can externally stabilize or de-stabilize the body during treadmill walking, for use in testing and assessing control of balance. This device will provide a means to experimentally manipulate the body's degree of stability, or apply small perturbations during walking. 2. To determine the relationship between the degree of body instability and characteristics of walking such as step length and width, step variability, and metabolic cost. We will test the hypothesis that these characteristics are interrelated due to the need to control balance by adjusting gait parameters and foot placement. 3. To perform a system identification of the human control system that stabilizes balance during walking. We will use the external stabilization/de-stabilization device to apply small noise-like perturbations to subjects walking on a treadmill, measure their associated control responses, and identify the systematic feedback responses that are used to maintain continuous stability during walking.